1. Field of the Invention
The invention is concerned with metallized polyimide substrates, and is specifically concerned with improving the adhesion between a polyimide substrate and metal (especially copper) layer for such purposes as permitting that metal layer to be soldered without danger of delamination.
2. Description of the Related Art
The insulated substrate of a printed circuit or a large-scale integrated circuit often is polyimide because of its stability at high temperatures, chemical resistance, and usefully low dielectric constant. A major concern is the adhesion of metal to the polyimide substrate, particularly at temperatures encountered in soldering operations. Efforts to improve adhesion primarily involve copper, because it is economical and almost always is the electrically conductive layer of a printed circuit or a large-scale integrated circuit.
A large number of patents and other publications concern the problem of improving the adhesion of metal to polyimide substrates. According to U.S. Pat. No. 4,152,195 (Baehrle et al.), earlier efforts to improve the adherence of a metal layer involved roughening of the surface of a polyimide substrate. Methods there mentioned include roughening by cathode sputtering, by chemical attack, and by generating electrical discharges in an oxygen atmosphere to partly burn the surface. The Baehrle method involved depositing a polyimide precursor onto a substrate, partially curing the precursor, vapor depositing a blanket layer of metal onto the partially cured precursor, and then fully curing the precursor.
Ruoff et al., "Improvement of Adhesion of Copper on Polyimide by Reactive Ion-beam Etching," IBM J. Res. Devel., Vol. 32, No. 5, pp 626-630 (September 1988), says that adhesion of copper to polyimide is largely due to mechanical interlocking and that one way to improve copper/polyimide adhesion is simply to roughen one surface or the other. Shown are electron micrographs of polyimide film after oxygen reactive ion-beam etching to produce a grass-like surface structure.
In U.S. Pat. No. 3,562,005 (DeAnqelo et al.), a metallic pattern is applied to an insulating substrate. When the substrate is a polyimide film, "bond enhancement entails heating the substrate and metallic pattern in an atmosphere conducive to the formation of oxides of the metal of the pattern . . . Such heating has been found to induce the growth of a metal oxide on the metallic pattern at the pattern-polyimide interface. Oxide growth in this manner drives the oxide into the polyimide to increase the pattern-polyimide bond to about 15 lbs./linear inch. Such oxide growth may be due to the permeability of polyimides to the oxide formation-conducive atmosphere" (col. 10, lines 16-27).
In EPO Pat. Appln. No. 86300070.9 (Publ. No. 0,187,706, Ho et al.), adhesion between an organic substrate and metal is enhanced by heating the substrate and then depositing metal atoms onto the heated substrate until at least a few atomic layers of continuous metal are formed on the substrate. When the organic substrate is a polyimide and the metal is copper, good adhesion is attained at 250.degree. C. because "the maximum amount of copper is found close to the surface of the polyimide", but at 300.degree. C., "the copper merely diffuses into the depth of the polyimide, and its concentration within a few hundred angstroms of the interface is insufficient for achieving good adhesion" (p. 11).
U.S. Pat. No. 4,806,395 (Walsh) discusses a number of known methods for improving adhesion of metal to polyimide substrates. In one of these, an initial chromium layer is sputtered onto polyimide film which is then covered with a sputtered layer of copper. While this can afford excellent adhesion, sputtering is expensive, especially in a continuous process, and when the resulting laminate is used for printed circuitry, it can be difficult to etch away the chromium sublayer along with the copper layer.
After discussing the prior art, the Walsh patent says: "Prior to the present invention, there has been no commercially viable method available for the direct coating of polyimide films with electrically conductive layers, without the use of an adhesive, exhibiting adequate properties for general use in electronic circuitry" (col. 4, lines 23-27). In the Walsh patent, at least one surface of a polyimide film is treated with an alkaline solution to provide a textured and hydrophilic surface that "is characterized by nodules or bumps, rather than pits as reported for other texturizing methods" (col. 5, lines 12-14). The textured surface "can be contacted with colloidal palladium in order to provide a catalytic surface for the deposition of electroless nickel or cobalt" (col. 5, lines 33-35), followed by electrolytically deposited copper. The textured polyimide can also be useful as a substrate for the fabrication of adhesive-based metal-clad laminates.
The Walsh patent says: "Copper and palladium are not useful as initial metal layers since they can catalyze degradation of the polyimide and lead to poor post solder float peel strength values due to thermal sensitivity of the final laminate structure." (Col. 10, lines 340-34)
U.S. Pat. No. 4,725,504 (Knudsen et al.) has substantially the same teachings as the Walsh patent, as does EPO Pat. Appln. No. 88301570.3 (Publ. No. 281,312, Walsh).
U.S. Pat. No. 3,981,691 (Cuneo) concerns the use of a layer of chromium to improve the adhesion of a copper layer to a polyimide layer and teaches that better adhesion between the polyimide and copper layers is attained when the chromium layer contains a significant percentage of oxygen atoms.
In U.S. Pat. No. 4,710,403 (Krause et al.), adhesion of metal to a polymer such as polyimide is enhanced by contacting a surface of the polymer with a reducing solution to leave a negative charge at the surface and then contacting the surface with an oxidizing solution having reducible metal ions such as copper ions, thus forming on the surface either a film or particles of that metal. When a copper film is formed, its enhanced adhesion "to the polymer is due to a mechanical anchorage of the metal caused by immediate diffusion of the metal complex just within the polymer surface where reduction occurs. Metal builds on top on this diffused region forming the thick, conductive, copper film" (col. 7, lines 13-19).
U.S. Pat. No. 4,775,556 (Krause et al.) contains substantially the same disclosure as does U.S. Pat. No. 4,710,403.